Environmentalists say plans to cut the Amazon reserve area would set environmental protection efforts back a long way, allowing loggers to cut down tropical forest roughly equal to the size of Belgium every year.
The bill, which is expected to be debated in Congress next week, would reduce the protected area of the Amazon forest from the present 80% of the total to 50%.
Environmentalists say that, if approved, the law would seriously accelerate the destruction of the rainforest.
The draft law reducing the protected area was passed by a parliamentary commission last week.
Loving the Amazon
"I am sure lawmakers will think and act like all Brazilians who love the Amazon," Mr Cardoso said, announcing that he would "fight" to maintain the environmental reserve area.
Environmentalists say plans to cut the Amazon reserve area would set environmental protection efforts back a long way, allowing loggers to cut down tropical forest roughly equal to the size of Belgium every year.
Since 1995 logging has been gradually reduced in Brazil from 30,000 square km (12,000 square miles) to roughly 17,000 square km in 1999.
The Amazon is the largest tropical rainforest on earth, and often described as the "lungs of the world".
The jungle is home to roughly 50% of the world's plant and animal species. Its area measures more than twice the size of France.
A small group of Brazilian congressmen who oppose the planned bill protested outside Congress on Tuesday by planting a tree.
Campaign against NGOs
Brazilian Defence Minister Geraldo Quintao on Tuesday joined a growing number of politicians who are critical of the role of non-governmental organisations in efforts to protect the Amazon.
"Why should we Brazilians be so submissive and allow these supranational groups to come here and intervene in our lives," Mr Quintao said.
Last week a senator in the upper house of Congress proposed that an investigative commission be formed to examine NGOs operating in the Amazon.
"Some NGOs work with million-dollar donations from companies with untrustworthy interests," said Senator Bernardo Cabral.
Such an investigation was welcomed by Garo Batmanian, head of the World Wildlife Fund in Brazil.
However, Mr Batmanian warned that some politicians were trying to create enemies out of environmentalists because public opinion was against the bill.
"They are trying to discredit the NGOs because they can't deal with the issues," Mr Batmanian said.